Constraining the axion portal with B → K ℓ + ℓ −

We investigate the bounds on axionlike states from flavor-changing neutral current $b\ensuremath{\rightarrow}s$ decays, assuming the axion couples to the standard model through mixing with the Higgs sector. Such GeV-scale axions have received renewed attention in connection with observed cosmic ray excesses. We find that existing $B\ensuremath{\rightarrow}K{\ensuremath{\ell}}^{+}{\ensuremath{\ell}}^{\ensuremath{-}}$ data impose stringent bounds on the axion decay constant in the multi-TeV range, relevant for constraining the ``axion portal'' model of dark matter. Such bounds also constrain light Higgs scenarios in the next-to-minimal supersymmetric standard model. These bounds can be improved by dedicated searches in $B$-factory data and at LHCb.